This invention relates to a solid composition and to a process for preparing a solid composition and in particular to a process for preparing a solid, water-soluble or water-dispersible composition containing a water-soluble material which is not film-forming and a film-forming water soluble material.
Film-forming polymers are used in a number of industries to provide a solid polymer medium within which a second non film-forming component may be supported. Typical of such applications is the casting of an aqueous solution of the film-forming polymer to form polymer sheets (tapes) or flakes.
Thus for example in WO 93/23999 there is disclosed a packaging for storing and releasing incompatible crop protection materials in which a chemical is xe2x80x9cencapsulatedxe2x80x9d or supported in a water-soluble polymer film.
Such processes typically involve as a first step dissolving a film-forming polymer in water to form an aqueous film-forming medium in which a material to be supported is dissolved or suspended. The film-forming medium is then for example cast onto a suitable substrate and dried to form a solid tape containing the material to be supported. Under certain conditions the tape may loose coherence during drying to form flakes. Alternatively, the film-forming medium can be dried to produce granules, agglomerates or powders.
We have found however that problems may arise when the non film-forming material to be supported is itself water-soluble, particularly if it is a strong electrolyte. Specifically, we have found that the presence of a water-soluble electrolyte in an aqueous solution of a film-forming polymer tends to interact adversely with the polymer at the relatively high polymer concentration required to provide adequate film-forming properties. As a result the film-forming polymer may be thrown out of solution as a rubbery deposit, and even quite small concentrations of water-soluble electrolyte may have a deleterious effect on the film-forming properties and homogeneity of the medium. The problem is exacerbated if the water-soluble electrolyte is hygroscopic such that even if a solid composition can be formed, it tends to pick up water causing the film-forming polymer component to become sticky.
According to the present invention there is provided a process for producing a solid, water-soluble or water-dispersible composition comprising a non film-forming material supported by a film-forming polymer wherein the supported material is a water-soluble material, which process comprises (i) preparing a film-forming aqueous medium containing (a) a film-forming polymer (b) a water soluble material which is non film-forming and (c) a water-miscible solvent in which the film-forming polymer is soluble and thereafter (ii) drying the film-forming aqueous medium to form the solid composition.
Whilst the process of the present invention may be applied to any water-soluble material which is not film-forming and which is suitable for being supported in a solid composition of a film-forming polymer, it is of particular relevance when the water-soluble supported material is a strong electrolyte and even more particularly when the water-soluble supported material, in its dry form, is hygroscopic. Typical strong electrolytes are salts, for example inorganic salts or salts of an organic acid or base. The scope of the present invention is not restricted to a water-soluble supported material having a specific utility, although it is illustrated herein with reference to a water-soluble supported material having utility in the agrochemical field, either as an active agrochemical or as an agrochemical adjuvant. Typical examples of water-soluble active agrochemicals which are strong electrolytes are salts of glyphosate, including without limitation the trimethylsulphonium salt, the isopropylamine salt, the sodium salt, the potassium salt and the ammonium salt and bipyridylium salts such as paraquat dichloride, glufosinate and fomesafen.
Typical examples of agrochemical adjuvants which are strong electrolytes are organic or inorganic salts such as ammonium sulphate. The process of the present invention provides a convenient method of obtaining a solid formulation of an agrochemical or an agrochemical adjuvant or an agrochemical formulation containing both active agrochemical and adjuvant having advantages in respect of handling, storage, transportation and reduced container contamination. Typical solid formulations of the present invention such as tapes or flakes provide a convenient delivery vehicle for the agrochemical or agrochemical formulation and may be arranged for example such that a single unit dose of agrochemical is contained in a unit dose package, for example in a conventional unit dose package or in water-soluble sachet packaging. If the process of the present invention is used to form a cast tape, the tape may be cut to provide a length corresponding to a desired dose. Furthermore we have found that the process of the present invention may be used to provide solid compositions containing a higher loading of agrochemical or agrochemical adjuvant than would be possible in the absence of water-miscible solvent. In certain circumstances the process of the present invention may be used to provide a solid composition containing an agrochemical formulation whose individual components are incompatible if used in the form of an aqueous liquid concentrate. Thus for example it may be possible to use a higher content of an adjuvant such as ammonium sulphate than would be compatible as an aqueous liquid concentrate formulation of an agrochemical.
In a preferred embodiment of the present invention, the solid composition additionally contains a solid filler.
According to a further aspect of the present invention there is provided a process for producing a solid, water-soluble or water-dispersible composition comprising a non film-forming material supported by a film-forming polymer wherein the supported material is a water-soluble material, which process comprises (i) preparing a film-forming aqueous medium containing (a) a film-forming polymer, (b) a water soluble material which is non film-forming, (c) a water-miscible solvent in which the film-forming polymer is soluble and (d) a solid filler and thereafter (ii) drying the film-forming aqueous medium to form the solid composition.
The solid filler is preferably a water-dispersible solid inorganic or organic filler such as calcium silicate, magnesium silicate (talc), sodium aluminium silicate, silica, mica, cellulosic fibre such as wood fibre, starch and diatomaceous earth. It is especially preferred that a highly adsorptive filler is used, for example a filler having a high surface area for example a surface area greater than 5 m2/g and preferably greater than 80 m2/g. As a specific example of a suitable filler there may be mentioned CALFLO E (CALFLO is a trade mark World Minerals), a calcium silicate filler having a surface area of about 100 m2/g.
The term xe2x80x9cfilm-formingxe2x80x9d polymer includes any polymer which is capable of providing film-forming properties in the presence of water. The film-forming polymer will generally be water-soluble but could also provide a film-forming aqueous medium in which the film-forming polymer is present in the form of a dispersion, and in particular a colloidal dispersion or in the form of a sol or in the form of a solution containing some dispersed material.
Suitable film-forming polymers include both synthetic and natural polymers such as polyvinylpyrrolidone, polyvinyl alcohol, partially hydrolysed polyvinyl acetate, modified polyvinylpyrrolidone such as a polyvinylpyrrolidone/vinyl acetate copolymer, polyethylene oxides, ethylene/maleic anhydride copolymer, methyl vinyl ether-maleic anhydride copolymer, water-soluble cellulose such as carboxymethylcellulose, water-soluble polyamides or polyesters, copolymers and homopolymers of acrylic acids, starches, natural gums such as alginates, dextrins and proteins such as gelatins and caseins. Mixtures of such film-forming polymers may also be used. Polyvinylpyrrolidone is an especially preferred film-forming polymer.
The water miscible solvent in which the film-forming polymer is soluble will vary depending on the nature of the film-forming polymer. Suitable solvents for use with polyvinylpyrrolidone or vinylpyrrolidone copolymers such as vinlyacetate/vinylpyrrolidone copolymers include alcohols, for example linear or branched chain primary or secondary alcohols containing from 1 to 6 carbon atoms, ethylene glycol, propionic acid, methylcyclohexanone, methylene dichloride, N-methyl-2-pyrrolidone, and diethanolamine. Ethanol is an especially convenient solvent in view of its ready availability and low cost. Suitable solvents for use with carboxycellulose include for example glacial acetic acid. Suitable solvents for use with other film-forming polymers will readily occur to those skilled in the art.
The film-forming aqueous medium is preferably formed by first dissolving the film-forming polymer in the relevant water-miscible solvent. To avoid unnecessary reduction in the film-forning properties of the polymer, it is preferred to dissolve the film-forming polymer in the minimum quantity of solvent. The solubility of the film-forming polymer in any given solvent may be readily determined, and illustrative proportions are given in the Examples. The solid filler, if used, is conveniently dispersed in the solution of the film-forming polymer in the solvent and the resultant mixture is then added to an aqueous solution of the water-soluble material which is not film-forming, for example to an aqueous solution of an agrochemical. Alternative orders of addition are equally acceptable but dispersion of the solid filler in the solution of the film-forming polymer in the solvent is generally easier than dispersion in the aqueous solution of the agrochemical.
The resultant film-forming aqueous medium is then dried to form a solid composition. The water-miscible solvent for the film-forming polymer is preferably volatile such that at least a major proportion of the solvent is removed with the water during drying. Whilst not being limited to any one particular theory, it is believed that preferred solvents such as ethanol may actually assist the removal of water (even in the presence of a hygroscopic water-soluble electrolyte), for example through the formation of an azeotrope.
The physical form of the resultant solid composition will depend on the exact manner of drying of the film-forming aqueous medium and a wide variety of processes may be used to provide a wide range of solid products. For example simple drying of the film-forming aqueous medium will generally form a powder or agglomerate. Greater control of the formation of a powder or granule product may be obtained by spray drying or freeze drying of the film-forming aqueous medium. The film-forming medium may be partially or wholly formed into fibres, for example by being extruded into a fast-moving stream of air, and the resultant solid composition may take the form of fibres or of a uniform particulate composition resulting from the breaking up of such fibres on further drying. The film forming aqueous medium can also be applied on an anti-adherent, rotating drum surface by means of a roller and subsequently dried by hot air to yield dry flakes. Alternatively the film-forming material may be cast in the form of a film onto a substrate, for example a conveyor belt, from which it is preferably removed after drying.
The casting of the film-forming aqueous medium onto a substrate may take place using conventional techniques such as tape casting. In tape casting, a film is formed on a substrate and the thickness is adjusted to that required using a device such as a xe2x80x9cdoctor bladexe2x80x9d which defines a pre-determined space between the surface of the substrate and the knife of the doctor blade. The substrate is conveniently a flat, planar surface but may also if desired possess indentations to provide appropriate corresponding patterning on the surface on the film. Similarly, the xe2x80x9cdoctor bladexe2x80x9d may have a contoured knife to provide corresponding patterning on the top surface of the film. In the extreme, the substrate may comprise one or more wells into which the film-forming aqueous medium is cast so that discrete pellets or tablets are formed on drying.
In commercial practice, it is normal to supply the film-forming medium from a reservoir and to form the film continuously, for example by the use of a moving belt as substrate or by movement of a reservoir and associated doctor blade relative to a stationary substrate. In commercial practice it is usually convenient to use a metal substrate although a plastics substrate may be used if desired.
The cast medium may be dried under atmospheric conditions but it is more conveniently dried at elevated temperature. In general it is sufficient to dry the cast medium at a temperature of from ambient to 100xc2x0 C., for example from 40 to 60xc2x0 C. It is to be understood that the drying process will not necessarily remove all traces of water and of the solvent for the film-forming polymer, and indeed a small proportion of residual water or solvent in the dry, cast product may have a beneficial plasticising effect. Typically levels of water in the range of 0.1 to 20% by weight are to be expected in the dry, cast product. Heating may be achieved for example by passing the cast medium into an oven or heated space or by applying heat to the substrate. Once the cast medium is dried, it may be removed from the substrate for subsequent use.
The cast medium may be removed from the substrate as a coherent sheet (a cast tape) and the coherent sheet may if desired subsequently be subdivided, for example by cutting, punching, or flexing to form flakes or shaped forms. Alternatively the proportions of the components of the film-forming medium, for example the content of the solid filler, may be selected such that the cast medium looses coherency during drying and cracks with the formation of flat flakes of product.
The thickness of the cast product, for example the cast tape or flakes, may be varied within wide limits according to the desired application. Typically the thickness of a cast tape or flakes varies between about 0.04 mm to 5 mm depending on the flexibility and other characteristics desired. If flakes are not formed directly, the dry tapes can be cut or fashioned to include a wide variety of shapes and designs, including for example discs, flakes, strips, tubes and spirals. The tape can be cut to provide a pre-determined metered dose of active ingredient which simplifies the formation of a dilute agrochemical spray for example. The tapes may also be embossed, corrugated or patterned to increase the surface area and may also carry printed information such as product and safety information.
For certain applications it may be desirable to protect the surface of the cast, dry product. The surface of the cast product may readily be protected by lamination or co-casting with a layer of water-soluble polymer which contains no active product and which may be the same as or different from the film-forming polymer. Alternatively, the cast, dry product may be housed in a water-soluble bag which may be manufactured from the same or different water-soluble polymer.
The proportions of the components of the solid composition formed by the process of the present invention may be varied widely depending on factors such as (a) the desired content of the active material in the solid composition (b) the process used to obtain the solid product and the desired properties of the aqueous film-forming medium and (c) the desired properties (such as dispersibility) of the resultant solid composition.
Thus for example if the film-forming aqueous medium is to be used to form a cast tape or cast flakes, a relatively higher film-forming level of polymer is likely to be required in the film-forming aqueous medium as compared with the film-forming aqueous medium used for spray-drying. Similarly a more coherent product is likely required if the final product is to be a cast tape rather than flakes. Such a coherent product is likely for example to require a relatively higher proportion of film-forming polymer as compared with the solid filler content. In general, sufficient of the film-forming polymer should be used to form a film-forming aqueous medium, by which is meant an aqueous medium having a suitable rheology and in particular a suitable viscosity for the drying process selected, for example for casting on a substrate. If there is insufficient polymer in solution, the aqueous medium will tend to run off the substrate and form too thin a film. If on the other hand too much polymer is present in the aqueous medium, it will not flow smoothly and the resultant film will not be self-levelling and uniform. The optimum concentration of polymer to provide an effective film-forming aqueous medium will vary depending on the exact nature and grade of polymer used but may be determined by simple and routine experimentation. Typical concentrations are illustrated in the Examples. Thus for example the concentration of the film-forming polymer in the film-forming aqueous medium is typically from 5 to 95% by weight, for example from 5 to 50% by weight.
As noted above, it is a particular advantage of the process of the present invention that a high loading of the water-soluble supported material may if desired be obtained in the solid composition. For example in favourable circumstances greater than about from 40% or 50% and even up to as high as 70% or more by weight of a water-soluble supported material such as an agrochemical active ingredient may be incorporated in the solid composition of the invention. This itself carries with it a further advantage in that potential problems of poor dispersion of the solid composition in water may be greatly reduced when a major proportion of the solid composition is the water-soluble agrochemical. It may thus for example be possible to use a film-forming polymer or other components which would otherwise give rise to dispersion problems if used at higher concentrations or if used in conjunction with water-insoluble components. It is of course possible to use lower proportions of water-soluble supported material, for example 20% or less by weight, if desired but some of the advantages of the present invention may not be so apparent in such products.
According to a further aspect of the present invention there is provided a solid, water-dispersible composition comprising a water-soluble agrochemical electrolyte, a film-forming polymer and a solid filler wherein the concentration of the water-soluble agrochemical electrolyte in the composition is greater than 60% and preferably greater than 70% by weight.
One skilled in the art will readily be able to determine appropriate proportions for each desired application but further detail is now given for the purposes of illustration only.
The proportion of polymer necessary to provide the required film-forming properties of the aqueous medium depends, at least in part, on the molecular weight of the polymer. We have found for example that polyvinylpyrrolidone is particularly suitable as a film-forming polymer which is commercially available in water-soluble or water dispersible grades having molecular weights in the range from about 8,000 to greater than 1,000,000 Dalton. A preferred grade of polyvinylpyrrolidone has a molecular weight in the range from 10,000 to 360,000 and in particular from 30,000 to 60,000. Polyvinylpyrrolidones having a molecular weight below about 30,000 can be used to form satisfactory cast tapes and flakes or other products, but a relatively large proportion of polymer is required to achieve a satisfactory film-forming aqueous medium with the consequence that the proportion of active supported material in the final product is undesirably reduced in favour of polymer. Similarly, polyvinylpyrrolidones having a molecular weight up to 360,000 may be used satisfactorily but the products tend to be sticky and, whilst the process of the present invention greatly reduces the risk of polymer coming out of solution in the presence of electrolyte, the use of very high molecular weight polymers might still give problems. As noted previously, any tendency to poor dissolution of the final product as a result of the use of a relatively high molecular weight polymer may be offset, at least in part, by the ability to increase the loading of the water-soluble active component and thus improve the dispersibility.
In general, we have found for example that for cast flake products, it is desirable to use a minimum of about 6% by weight of polyvinylpyrrolidone having a molecular weight of between about 30,000 and 50,000 to achieve coherent flakes. There is no real upper limit to the proportion of polyvinylpyrrolidone which may be used but generally there is little advantage in a product having high levels of film-forming polymer and low levels of supported material. The proportion of film-forming polymer in the final product is thus typically from 6% to 50% by weight.
We have found that the use of a solid filler provides a number of advantages and in particular avoids excessive stickiness in the product if for example a hygroscopic water-soluble electrolyte is supported by the film-forming polymer. Typically it is desirable for this reason to use at least 9% by weight and preferably at least about 20% by weight of solid filler in the final product. There is little advantage in a product having excessively high levels of solid filler, and the proportion of solid filler in the final product is thus typically from 9% to 50% by weight, for example from 20% to 30% by weight.
If desired, other components may be added to the film-forming aqueous medium. Thus for example it may be desirable, particularly if the cast product is to be a cast tape (a film), to include a plasticiser to improve the flexibility of the cast product. Suitable plasticisers include glycerols, C2 to C6 glycols and polyglycols such as polyethylene glycol, dialkyl phthalates such as dioctyl phthalate, sorbitol and triethanolamine or mixtures thereof. In addition to improving the flexibility of the product a plasticiser may also have an advantageous effect on the rate of dispersion of the dry, cast product in water. The proportion of plasticiser is preferably within the range 0 to 80% by weight, for example from 5 to 30% by weight relative to the film-forming polymer.
Surfactants may be added to the film-forming aqueous medium both to enhance the rate of dispersion of the dry product in water and also to affect the surface tension properties of the film-forming aqueous medium relative to a substrate on which it is cast. Thus for example a wetter may be added to ensure wetting of the substrate, for example if a plastics substrate is used. If it is desired to produce cast tapes rather than flakes, surfactants may also be added which modify the surface tension of the wet cast film and ensure that on drying the film reduces in thickness with minimum shrinkage in the plane of the substrate on which it is cast. A wide variety of surfactants may be used for these purposes and suitable examples will occur to one skilled in the art. Solid surfactants may be present in relatively high loading in the cast product and may be used for example to provide adjuvant properties in the final application, for example as a wetter in a spray solution for agrochemical use.
An antifoam agent may be added to prevent excessive aeration during mixing of the film-forming aqueous medium. A viscosity aid may be added if desired to modify the viscosity of the film-forming aqueous medium, for example to minimise any settling of the solid filler within the thickness of the wet film during drying. Suitable viscosity-modifying aids include alginates, starch, gelatin, natural gums, hydroxyethyl cellulose, methyl cellulose, silica and clays.
According to a further aspect of the present invention there is provided a solid water-soluble or water-dispersible composition whenever prepared by a method according to the present invention.
The invention is illustrated by the following Examples in which all parts and percentages are by weight unless otherwise stated.